<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" lang-="en">

/* Media query */
@media screen and (max-device-width:600px), screen and (max-width:600px) {

/* .class { attribute: value !important;} */
.image { width: 100%!important;}

<table role="presentation" width="100%" border="0" cellspacing="0" cellpadding="0">
<!-- The image we want to change to 100% width with the class="image" --> 
<img src="imagelink.png" width="100" height="auto" style="display:block; height:auto;" class="image">

External CSS

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External JavaScript

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